Membrane-covered measuring probes, specifically amperometric sensors are increasingly used for monitoring biological and biochemical processes as well as in the beverage and brewing industries, for determination of gases and/or nonionic compounds contained in a measured medium, for instance volatile ingredients of the measured medium or gases, specifically oxygen. To achieve an optimum function of such measuring probes, for instance a short response time, high sensitivity, low detection limit and good long-term stability, it is necessary to ensure that, e.g., pressure differences caused by temperature fluctuations between the measured medium and the interior of the measuring probe surrounding its sensitive parts will be extensively avoided or compensated for. Especially necessary for such measuring probes is to allow steam sterilization, so that the pressure differences occurring during steam sterilization will not affect the function of the measuring probe, i.e., measures are to be taken by which a deformation of the membrane due to high pressure differences between the measured medium and the probe interior will be prevented. In the case of measuring probes with a nonreinforced gas-permeable membrane, for instance as described in U.S. Pat. No. 2,913,386, a sufficient pressure stability can be achieved only with high equipment expense. To counteract the undesirable membrane deformation, various approaches were taken and the following measures applied:
a) pressure equalization using suitable devices; PA1 b) maintenance of a pressure gradient from the measured medium toward the interior of the measuring probe; PA1 c) reinforcement of the membrane.
The pressure equalization cited under a) is accomplished, for instance with the oxygen electrode described in U.S. Pat. No. 4,252,627 featuring an electrolyte-filled interior, in such a way that the probe housing is provided with an air gap via which the probe interior can communicate with the measured medium. A swift and exact pressure equalization can be achieved thereby; but it risks that volatile compounds can unimpededly penetrate the probe interior from the measured medium. This may cause a change of the electrolyte composition or, if no electrolyte is present, a change of the gas phase present in the probe interior, with the result of possible measuring errors in both cases.
The pressure equalization mentioned under a) may also be accomplished with the use of a flexible membrane or with the aid of a fluid drop, as described in U.S. Pat. No. 4,455,213, or by means of a piston moving in the probe housing. Besides, the pressure equalization can be accomplished by means of a valve which opens at a specific overpressure in the probe interior. This allows keeping the interior pressure within certain limits, but providing a valve causes a considerable equipment expense and, additionally, counter-acts a miniaturization of the probe, which for numerous applications is desirable.
The adjustment of a pressure gradient as mentioned under b) can be accomplished, e.g.. in that the probe interior is open toward the atmosphere causing the membrane to be forced on a backing, by the pressure of the measured medium. The membrane suffers then hardly a deformation, provided the roughness of the backing required for maintaining an electrolyte film is not selected excessively high and the pressure does not exceed a specific maximum value. In the case of probes where the interior is gas-filled, a satisfactory function is given only if the measurements will not be influenced by the penetration of atmospheric matter.
From the above comments it is evident that a sufficient pressure stability, and thus the prevention of an undesirable membrane deformation, can be accomplished with the measures cited under a) and b) only with considerable equipment expense. This applies to all measuring probes equipped with a single, nonreinforced gas-permeable membrane.
Less expense is required by the reinforcement of the membrane mentioned under c). The reinforcement may be accomplished, e.g., by embedding a backing material, for instance a netting or perforated panel from rugged material, for instance stainless steel, such as known from U.S. Pat. No. 3,718,562. This U.S. patent document concerns an electrode arrangement with a membrane made of selectively permeable material, for instance silicone caoutchouc, in which a porous interlacing is embedded as reinforcement. The reinforcement material is preferably a netting from an organic polymeric material or a steel net. Membranes of that type, especially when made of silicone caoutchouc are chemically not resistant, especially not to aggressive cleaning agents, and contaminate easily, for instance by bacterial growth when used in biological or biochemical processes, thereby impeding access of the gases or volatile compounds to be determined, to the sensitive parts of the measuring probe accommodated in its interior. When using, instead of a silicone caoutchouc membrane characterized by a high permeability, a membrane of fluorized hydrocarbon, for instance polytetrafluoro ethylene (PTFE) Teflon, it will be characterized by a very good chemical resistance, but have the disadvantage that its permeability to the gases to be determined will be by about two powers of ten lower than that of silicone caoutchouc.
Known from U.S. Pat. No. 3,718,562 is a polarographic measuring probe with a membrane consisting of two layers, where the first layer arranged on the medium side is formed by a material featuring a permeability that nearly equals that of silicone caoutchouc, while the second layer facing toward the probe interior is formed by a hydrophobic material whose permeability to gases and water vapor is considerably lower than that of the first layer. These layers are not interconnected; but they are fixed in the probe housing in such a way that a penetration of electrolyte solution and/or gases to be determined, into the space between the layers, will be prevented. This membrane structure achieves that the gas to be determined, which is used up on the cathode, will be much quicker replenished than is the case with a membrane formed of one layer, a so-called single membrane. Avoided this way, with the membrane formed of two layers, over long periods of time is an impoverishment of the gas to be determined, which yields exact measuring results and a short response time of the measuring probe equipped with this membrane. Unfavorable, however, is that the first layer arranged on the medium side, due to its material properties, is chemically little resistant and, when used in biological and biochemical processes, may easily be contaminated by bacterial growth.
To avoid the aforementioned disadvantages, the Swiss patent document . . . (application No. No. 02 503/88-9) proposed to make a membrane of two layers where the layer on the medium side is formed of a chemically resistant material and is insensitive to bacterial growth, and to integrate a backing material in the other layer formed of a material which in comparison to the material of the first layer possesses an increased permeability. The measuring probe equipped with this membrane favorably is distinguished from the aforementioned one by its chemical resistance, its insensitivity to contamination by bacterial growth, and by the stability of the membrane, but it is insufficiently insensitive to larger pressure differences between the measured medium and the probe interior, in case of applications requiring a steam sterilization of the measuring probe.
Therefore, the problem underlying the invention is to provide a measuring probe for determination of gases and/or nonionic compounds contained in a fluid or gaseous measured medium which is insensitive to pressure differences between the measured medium and the probe interior such as caused by temperature fluctuations, specifically in the course of steam sterilization, also in the range of &gt;10 bars, and is additionally chemically resistant and dirt repellent.
This problem is inventionally solved by a measuring probe for amperometric determination of gases and/or non-ionic compounds contained in a measured medium, with an internal body accommodated within a probe interior defined by the probe housing, in which internal body an element sensitive to the gases and/or non-ionic compounds to be determined is integrated, and with a membrane terminating the probe interior housing, with the part of the probe housing supporting the membrane forming a membrane module which can be connected with an electrode shaft supporting the internal body, characterized in that the membrane consists of at least three layers where a first layer of chemically resistant material, facing in built-in condition toward the probe interior, is connected with a second layer on the surface of the first layer facing toward the probe interior, which second layer has in comparison with the first layer a greatly increased permeability to the gases and/or non-ionic compounds to be determined, while on its surface opposite the first layer there is a third layer arranged which completely covers the second layer and is formed by a material at least nearly equivalent to that of the first layer with regard to chemical resistance and permeability to the gases and/or non-ionic compounds to be determined, and in that in the membrane module there is a pressure equalization system integrated which at a pressure gradient occurring in operating condition between the measured medium and the probe interior is deformable in the direction of the pressure gradient.
Achieved by combining a membrane formed of at least three layers, where the middle layer, formed of a material which as compared to the material of the two outer layers displays a greatly elevated permeability to the gases and/or nonionic compounds to be determined--for simplicity hereafter called measured goods--and the two outer layers are chemically resistant and dirt-repellent, with a pressure equalization system integrated in the probe housing, is that a deformation of the membrane--also at pressure differences between the measured medium and the probe interior of approximately 20 bars--will be avoided practically completely, that a replenishment of measured goods used up at the cathode will be safeguarded without specific measures or equipment expense, thus avoiding an impoverishment of measured goods in the probe interior, and that damage to the membrane by the attack of aggressive chemicals as well as its contamination by bacterial growth will be prevented.
Another embodiment avoids a mutual separation of the layers while assuring an especially good stability of the membrane.
The selection of the material for the first and third layers and their combination with the second layer assures, for one, a high chemical resistance of the membrane surface which in operating condition makes contact with the measured medium and, for another, a sufficient replenishment of measured goods, due to the permeability differences between the materials chosen for the first and second layers, thereby avoiding an impoverishment of the measured goods in the probe interior in the case of prolonged measuring duration, and thus precluding an adulteration of measuring results.
Achieved by integration of a backing material in the middle, second layer, is an especially good form stability of the membrane, with a net from stainless steel being especially preferred.
The rigid fixing of the membrane and its sealing in the probe housing, causes the exchange of the gases and/or nonionic compounds to be determined to take place at the operating condition of the measuring probe exclusively via the membrane, while all other areas of the measuring probe are hermetically sealed from the measured medium and the surroundings, thus effectively precluding a penetration of contaminants, for instance of volatile substances contained in the measured medium or of atmospheric matter from the surroundings. This effect is greatly amplified yet by the embodiment, which additionally enables a simple measuring probe assembly.
The pressure equalization system enables a swift elimination of a pressure gradient between the measured medium and the probe interior in both directions.
Other embodiments, for one, assure a high insensitivity to the attack of aggressive chemicals, such as partly used in cleaning, and to the effect of elevated temperatures such as occurring during steam sterilization.
Besides, unevennesses or cracks in the surface of the probe housing, that might cause a contamination of the probe housing and bacterial growth, are avoided.
The measuring probe of the initially cited type can be used to particular advantage for the amperometric determination of oxygen in gas mixtures or fluids. But it can be used also for the determination of chlorine or hydrogen in gas mixtures and fluids.